Bovine leukemia virus (BLV) is closely related to human T-cell leukemia virus types 1 and 2 (HTLV-1 and -2) and is the etiological agent of enzootic bovine leukosis (EBL), which is the most common neoplastic disease of cattle [Non-Patent Literature 9]. Infection with BLV can remain clinically silent, with cattle in an aleukemic state. It can also emerge as a persistent lymphocytosis (PL), characterized by an increased number of B lymphocytes, or more rarely, as a B-cell lymphoma in various lymph nodes after a long latent period [Non-Patent Literature 9].
In addition to the structural and enzymatic Gag, Pol, and Env proteins, BLV encodes at least two regulatory proteins, namely Tax and Rex, in the pX region located between the env gene and the 3′ long terminal repeat (LTR) [Non-Patent Literature 9]. Moreover, BLV contains several other small open reading frames in the region between the env gene and the tax/rex genes in the pX region. These encode products designated as R3 and G4 [Non-Patent Literature 10]. BLV has two identical LTRs, which possess a U3 region, an unusually long R region, and a U5 region; these LTRs only exert efficient transcriptional promoter activity in cells productively infected with BLV [Non-Patent Literature 9]. BLV can integrate into dispersed sites within the host genome [Non-Patent Literature 11] and appears to be transcriptionally silent in vivo [Non-Patent Literature 12]. Indeed, transcription of the BLV genome in fresh tumor cells or in fresh peripheral blood mononuclear cells (PBMCs) from infected individuals is almost undetectable by conventional techniques [Non-Patent Literatures 12, 13]. In situ hybridization has revealed the expression of viral RNA at low levels in many cells and at a high level in a few cells in populations of freshly isolated PBMCs from clinically normal BLV-infected animals [Non-Patent Literature 14]. It appears that BLV provirus remains integrated in cellular genomes, even in the absence of detectable BLV antibodies. Therefore, in addition to the routine diagnosis of BLV infection using conventional serological techniques such as the immunodiffusion test [Non-Patent Literatures 15-18] and enzyme-linked immunosorbent assay (ELISA) [Non-Patent Literatures 17-20], diagnostic BLV PCR techniques that aim to detect the integrated BLV proviral genome within the host genome are also commonly used [Non-Patent Literatures 17-19, 21-23].